Where were the convicts imprisoned before transportation?

Answer 1

Many of them were held in former convict transport ships and other ships converted to what were termed Hulks. These ships had accomodation built into them and had masts, etc removed. The British generally did not imprison convicts (although there were prisons, of sorts) preferring to humiliate people for minor indescetions and execute them (by hanging) for even quite minor crimes - such as a theft in excess of a days pay.

Transportation (whether by ones own means or after having been sold to traders who would then sell them to plantation owners in Virginia) was usually a coomutation of a death sentence. In later times, though, by the time tranpirtation to Australia, instead of America, was occurring a penalty of transportation was often levied directly